Solenoid operated valves have found widespread usage in on-board vehicle applications for controlling hydraulic pressure and fluid flow in automatic shifting power transmissions on the vehicle. Conventional solenoid valve structures include a solenoid that receives an electrical current signal, an armature that moves in response to the signal via magnetic force, and a spool that operates in response to the armature movement to change a pressure output of the valve. A pin acts as a mechanical interface between the armature and the spool. A spring force acts on the armature as well; as a result, the position of the armature and spool, and therefore the valve pressure, depends on the counterbalance of forces in the spring, the magnetic forces on the armature, and the hydraulic pressure. Normally, the spool is continually engaged with the armature via any pressure imbalances on the spool, the spring force, or both. The spool therefore meters fluid out of the valve based on the armature position.
Occasionally, contaminants may enter the valve. To loosen the contaminants so that they can be flushed out of the valve, a dithered signal is applied to the armature to continually oscillate the armature, and therefore the spool. However, proper valve operation requires the armature and spool to be in constant contact. Low operating pressures within the valve may cause the spool to lose contact with the armature, preventing dither in the armature from being transferred to the spool. As a result, any future movement by the spool must first overcome static friction, causing the spool to unstick itself in an uncontrolled fashion, creating undesirably rapid pressure change at the valve output.
Also, currently known valve structures require tight manufacturing tolerances. The alignment of the pin, armature and spool are particularly delicate and must be perfectly linear and centered to ensure proper valve operation. If any one of these components is even slightly out of alignment, the valve will fail. The pin, in particular, must be perfectly aligned with the spool; however, the length of the pin and the spool causes any misalignments in the pin to be magnified at the spool. The many components in current valve assemblies makes it challenging to manufacture and introduces greater opportunities for potential valve malfunctions.
There is a desire for a solenoid valve assembly that operates more reliably and also has a simpler construction than currently known valve assemblies.